1. Field of the Invention
The present invention relates to a printing apparatus, and particularly to a technology in which paper can smoothly be conveyed and a technology in which a long life and high reliability are obtained in the printing apparatus.
2. Description of the Related Art
Currently, a thermal printer is used to print a receipt with a register in a restaurant and a store. Usually single-side printing is done to the receipt, and a large amount of receipt paper is used in the case of printing a large amount of information. Therefore, sometimes a double-side simultaneous printing thermal printer is used to print the information on the paper as much as possible.
In the thermal printer which simultaneously carries out printing on the both surface sides of thermal recording paper, for example, Jpn. Pat. Appln. KOKAI Publication No. 11-286147 discloses a double-side printing thermal printer including two platen rollers and two thermal heads. The thermal recording paper passes between the thermal head and the platen roller, and the printing is done on the thermal recording paper by heat applied to the thermal head.
In such kind of double-side printing thermal printer, the first platen roller and the second platen roller are rotated at the same speed while being synchronous with each other. The first thermal head carries out the printing on one of the surfaces of the thermal recording paper by the passage of the thermal recording paper between the first platen roller and the first thermal head. The second thermal head carries out the printing on the other surface of the thermal recording paper by the further passage of the thermal recording paper between the second platen roller and the second thermal head.
In the conventional double-side printing thermal printer, when the first platen roller differs slightly from the second platen roller in a feed speed, looseness of the thermal recording paper is generated between the pair of platen rollers, or tension is excessively applied to the thermal recording paper, which possibly results in a problem with print quality. Therefore, it is necessary to accurately manage an outer diameter and the feed speed of each platen roller. However, because the platen roller is made of a rubber material having elasticity, there is a limitation to the accurate management of the outer diameter and feed speed in the platen roller.
In some kinds of the printing apparatus, a first printing unit located on the downstream side of a paper conveyance path in the paper conveyance direction and a second printing unit located on the upstream side are provided, the paper is entrained between the first and second printing units to simultaneously carry out the printing on the one surface side of the paper by the first printing unit and the printing on the other surface side of the paper by the second printing unit.
The first printing unit includes a first thermal head which is a printhead and a first platen roller which conveys the paper. The first platen roller is arranged to face the first thermal head through the paper conveyance path. The second printing unit includes a second thermal head which is a printhead and a second platen roller which conveys the paper. The second platen roller is arranged to face the second thermal head through the paper conveyance path (for example, see U.S. Pat. No. 6,784,906).
Because the double-side printing is simultaneously started while the paper is entrained between the first printing unit and the second printing unit, the printing start positions are displaced between one surface and the other surface of the paper, which generates waste.
Therefore, the paper is reversely conveyed by an amount in which the waste is generated, the printing is started by the second printing unit when the paper is normally conveyed, and the printing is started by the first printing unit to eliminate the waste at the time the printing start portion reaches the first printing unit.
However, in the conventional techniques, in order to prevent the conveyance trouble caused by the looseness of the paper between the first printing unit and the second printing unit, the paper feed speed of the platen roller of the first printing unit is set faster than that of the platen roller of the second printing unit to apply the tension to the paper between the first printing unit and the second printing unit.
Therefore, when the paper is reversely conveyed such that the printing start positions are aligned with each other, the reversal feed amount of the paper by the platen roller of the first printing unit becomes larger than that of the platen roller of the second printing unit, and the looseness is generated in the paper, which causes a conveyance trouble.
Furthermore, because the number of printing units is increased to increase resistance against the paper conveyance, necessary power is increased, which results in a problem that breakage or wear of each component easily occurs.